In the telephone system today, which conveys voice communications and digital data, it is necessary to convey a number of channels, each carrying a discrete conversation, over a single pair of conductors or a fiber optic cable. To accomplish this, the channels are multiplexed at a particular frequency depending on the number of channels to be applied to a conductor pair. For example, in the instance where a PBX (public branch exchange) is installed in an office building, analog signals for each conversation or channel are coupled to a PBX switching and multiplexing unit in the building. Here, the signals are digitized by an A/D converter and multiplexed utilizing time division multiplexing, and output as a serial, digital data stream on a single pair of conductors at a frequency of 1.544 Mhz. At this frequency, up to 24 channels may be conveyed on the single pair of conductors, with this frequency and number of channels designated as a T1 signal. This T1 signal, or 24 digital channels at 1.544 Mhz, is routed to a switching station outside the building or to a central office proximate the destination, where the discrete 24 channels are demultiplexed and directed to their destination as an analog signal. In the instance where the call is a long distance call, connections are made to an area network, which includes a multiplexer that multiplexes the various channels at a frequency of 44.736 Mhz, and which will carry the equivalent of 24 T1 signals, or 576 discrete channels over a single pair of conductors. This frequency and number of channels is designated as a T3 signal. This particular scheme of multiplexing discrete channels at higher frequencies continues to very high data rates which are suitable for overseas communications between continents over fiber optic conductors, and which operates in the gigahertz range, and conveys in excess of 25,000 channels over a single fiber optic light guide.
In order to separate the discrete channels, and in one scheme designated as D3/D4 or Superframe, which was introduced in 1973 and which enjoys wide use today, the T1 signals are divided by framing bits into frames of 192 bits, or 8 bits per frame for each of the 24 channels, with the 193rd bit being a framing bit. The frames are further divided into sequences of Superframes, which is a sequence of 12 frames, with the Superframes being separated by framing bit sequences designated FT, which are used for synchronization, and FS, which are used for data link communication between terminals or for synchronization.
A number of problems, or errors in the data may occur in the circuitry or the pairs of wires conveying the T1 or T3 signals. Among them are (1) wander, or the difference in position where a bit happens to be and where it should be, (2) jitter, which is similar to wander, (3) drift, which is the difference between one reference clock frequency and another clock frequency, (4) slips, which are controlled repetitions or deletions of frames in order to make up for differences in digital terminal clock rates. These problems are related to properties of the signals, although other problems may occur, such as loss of signal or signal amplitude being too low or too high. Further, format or protocol problems may occur, such as incorrect or loss of framing bits, or more than 15 digital zeroes in a row. Further yet, problems may occur in transmission lines between terminals and switching stations, such as installation of cables having the wrong impedance, bad ground connections, and the like.
In order to diagnose and determine problems in a data transmission line as described, Phoenix Microsystems located in Huntsville, Ala., has developed portable test sets such as the 5575 Microbert series, which is typically a small diagnostic test set designed to be carried by a technician and which serves to perform a number of tests including bit error rate tests (BERT) and also checks signal parameters such as amplitude, frequency, etc. One version of this test set, and which is generally representative of this series, is provided with a four line by forty column liquid crystal display for displaying one of up to 24 menu selections for configuring the test set and for indicating certain types of failure modes of the signal, 14 status/alarm LED indicators, a 24 key keypad, and a number of switches used to configure the test set in order to match protocols of the signal line and/or circuitry to be tested.
Due to the complexity of the test requirements for a test set of digital communication lines, considerable familiarity with digital communications systems and protocols is required in order for a technician to operate a bit error rate tester correctly and with some measure of efficiency. It is well known that help keys and help menus are of only limited value, and that test instruments are becoming increasingly complex. Further, the technological level of users of the equipment does not generally keep pace with advances in the technology. Also, when in doubt, technicians and others operating test equipment tend to experiment with the equipment in the hopes of hitting on proper setup or test procedure, and generally only read a service or instruction manual as a last resort. This type of experimentation is inefficient, and can occasionally cause damage to the test set or to the circuitry under test.
In accordance with the foregoing, it is an object of this invention to provide a system having access to all signal inputs, to all status and selection modes of the test set and results of attempted tests, and a method inherent in the operation of the circuitry for guiding the technician to set up the test set properly so that the test set can analyze the telecommunications data and display the "Why?'s" of the problems and the solutions to such problems.